Processor speeds of computers continue to increase. Devices with which the processor communicates often do not operate at such high speeds. For example, static random access memories (SRAMs) often operate at almost as high a speed as the processor, but dynamic random access memories (DRAMs) operate at a slower speed. Dynamic random access memories possess advantages to static random access memories. For example, static random access memories require more space than dynamic random access memories.
Rambus Inc. of Mountain View, Calif. has technology that allows DRAMs and controllers or processors to transfer data at a high frequency, such as 600 megabytes per second and above over a Rambus Channel, a narrow byte-wide data bus. Attention is directed to the following patents assigned to Rambus, Inc., which are incorporated herein by reference: U.S. Pat. No. 5,680,361 to Ware et al.; U.S. Pat. No. 5,663,661 to Dillon et al.; U.S. Pat. No. 5,537,573 to Ware et al.; U.S. Pat. No. 5,499,385 to Farmwald et al.; U.S. Pat. No. 5,499,355 to Krishnamohan et al.; U.S. Pat. No. 5,485,490 to Leung et al.; U.S. Pat. No. 5,446,696 to Ware et al.; U.S. Pat. No. 5,432,823 to Gasbarro et al.; U.S. Pat. No. 5,430,676 to Ware et al.; U.S. Pat. No. 5,390,308 to Ware et al.; U.S. Pat. No. 5,355,391 to Horowitz et al.
An alternative to Rambus has been developed by memory chip makers. The synchronous link DRAM (SLDRAM) is an alternative to double-data-rate (DDR) and Direct Rambus DRAM.
The SLDRAM is known in the art. The SLDRAM, formerly known as SynchLink, is designed for computer main memory in mobile, desktop, workstation, and server systems. It is designed to reduce a speed bottleneck in accessing memory from a processor. The SLDRAM project attempts to solve a memory system problem that will become more acute in newer systems. DRAM memory chips do not have enough bandwidth for getting the data on or off the memory chips. To solve this problem, manufacturers have been using many chips in a wide array to get the speed up to what their system needs. However, new DRAM chips will have increasingly higher capacities, so that there will be so much DRAM capacity in the wide array of chips needed for getting the speed, that the price of the DRAM capacity raises the price of the computer. For lower price or entry-level computers and workstations, this price may be excessive. Unnecessarily large memory would exist in base configurations. Although new software uses more memory, that memory usage is not growing as fast as DRAM density, and this mismatch may result in overly expensive computers.
SLDRAM addresses this problem by using a new architecture for communicating with the DRAMs, with two highly optimized buses. This allows increasing the DRAM bandwidth significantly. SLDRAM adds pipelined transfer protocol for increased advantage of bandwidth. Attention is directed to the SLDRAM White Paper of 29 Aug. 1997, which describes SLDRAM in greater detail.
SLDRAMs are synchronously linked to processors. To provide high speed access to the memories, as processor speed increases, lengths of circuit traces should decrease.
It is known to use optical waveguides as interconnects from integrated circuit to integrated circuit. See, for example, U.S. Pat. No. 5,119,451, which is incorporated herein by reference. Various R&D efforts have taken place in an attempt to develop optical interconnect technology for short-haul data communications applications such as for communications between boards, backplanes, and intra-boxes. See, for example, “Lighting the Way in Computer Design,” IEEE Circuits & Devices, January 1998.